present invention relates generally to conditioning elements for telecommunications transmission lines and, more particularly, to remote channel unit having a trans-hybrid loss compensation circuit used with digital communication transmission facilities. Such facilities include a central office which may transmit data signals over transmission lines to remote terminals and customer premises. The signals may be sent over two pairs of transmission lines in a digital format or differentially on two conductors, known as the Tip-Ring Pair.
The Bell telephone system in the United States, for example, has a widely utilized Digital "D" multiplexing pulse code modulation systems. A "D" channel bank, for example, commonly provides multiple DS-1 signals that are carried on a T-1 transmission system. One pair of cables is provided for each direction of transmission.
Signals which are sent via digital carrier transmission lines from the central office reach a remote terminal before reaching the customer premises. The remote terminal then converts the digital signals to an analog signal which may then be an intelligible signal for the telephone.
Thus, in the remote terminal, the digital data signals, sent over the T-1 lines, are converted to analog telephone signals and then supplied to a customer loop telephone line. The analog telephone signals may then be received by the customer premises, which may include telephones and, in some cases, PBX units. Conversely, the remote terminals receive analog telephone signals from the telephones and PBX units and convert them to digital data signals, which then can be transmitted over the T-1 transmission system.
The length of the telephone line between the remote terminal and the PBX or telephone in the customer premises may vary substantially. The length of the line affects the impedance of the line and, thus, for example, the amplitude of signals between the remote terminal and the telephone.
Many transmission lines include a hybrid circuit that interfaces a two-wire transmit transmission line and a two-wire receive transmission line (a four-wire circuit) to the Tip-Ring pair. Part of the signal on the receive transmission line may be reflected back to the transmit transmission line. Thus, a common practice is to invert the received signal and add it to any transmit signal received from the telephone lines (to effectively cancel out "reflected" signals). The level of the transmitted signal is compared to the receive signal and represented in dB: dB=20 Log Transmit Voltage/Receive Voltage.
In presently available equipment, such a trans-hybrid converter may work well if the telephone line is relatively short. However, if the telephone line is fairly long, the reflected signal may exhibit a phase shift and have decreased in amplitude substantially. As a result, the inverted signal may be larger (or smaller) than the reflected signal and it may be more or less than 180.degree. out of phase with the reflected signal. Consequently, reflected signals, or noise, will be introduced into the transmission line system.